It is desirable to monitor the output current as well as the output voltage of a power supply for varied purposes, such as current regulation, overcurrent limit protection or similar desirable objectives. This generally requires dual sensing devices; one sensing the output voltage and another sensing the output current. Voltage sensing devices draw very little power and have a limited effect on the voltage signal which they are sensing.
Current sensing devices, on the other hand, are usually connected in series with the path of the current to be sensed and hence must conduct the full current flow. In order to limit the power dissipation of the current sensing device, its resistance is usually extremely low. Hence, a voltage sensed across the current sensing device is a very low level signal. Measurements of these very low level signals have inherent noise and stability problems because of the necessary sensitive detecting devices.